1. Field of the Invention
The present invention relates to a die cushion controlling apparatus and a die cushion controlling method which controls an operation of a cushion pad synchronously with an operation of a slide of a press machine.
2. Description of the Related Art
In a press machine, a die cushion apparatus (hereinafter merely referred to as a “die cushion”) is provided for controlling folds in a throttling work. A conventional die cushion generates a cushion pressure while driving to raise or lower a cushion pad by using a hydraulic pressure or an air pressure. In order to raising throttling workability of the press machine and prevent a work from being broken or strained, it is necessary to control the cushion pressure of the die cushion in high accuracy, and particularly, it is necessary to control the cushion pressure at the time of lowering operation of the cushion pad in high accuracy.
The die cushion using only the air pressure cannot control the cushion pressure in high accuracy at the time of operating the cushion pad. The die cushion using the hydraulic pressure can control the cushion pressure in high accuracy at the time of operating the cushion pad under the control of a hydraulic pressure. However, there is a drawback that the structure of a hydraulic apparatus is complicated, and severe maintenance and management is required. Therefore, recently, a die cushion having an electric servomotor which has a simple structure and which does not need severe maintenance and management is noted.
In Japanese Patent Application Laid-Open No. 10-202327, a control technology of a die cushion having a rotary electric servomotor is disclosed. FIG. 18 is a view showing a conventional press machine and its control system.
In the press machine shown here, a slide 2 is coupled to an eccentric portion of a crankshaft in a slide drive mechanism 1. The slide 2 is raised or lowered in response to a rotation of the crankshaft. An encoder is provided in the crankshaft, and a signal is outputted from the encoder to a controller 100 in response to the rotation of the crankshaft. The controller 100 obtains a position of the slide 2 by using this signal.
Also, in the die cushion shown here, the output shaft of the servomotor 16 is coupled to a screw portion 112b of a ball screw 112, and this screw portion 112b is screwed into the cushion pad 11. When the screw portion 112b of the ball screw 112 rotates in response to the rotation of the servomotor 16, the cushion pad 11 is raised and lowered along the screw portion 112b. The servomotor 16 is provided with an encoder and a signal is outputted from the encoder 19 to the controller 100 in accordance with the rotation of the servomotor 16. The controller 100 obtains a position of the cushion pad by using this signal.
At an initial time of one stroke operation of the slide 2 from a top dead point, the controller 100 controls the position of the cushion pad 11 in accordance with the position of the slide 2. By this control, the cushion pad 11 is lowered at a lower speed than a lowering speed of the slide 2 and operated so that an upper die 3a is contacted with a work 4 at a predetermined position. When the upper die 3a is contacted with the work 4, the cushion pad 11 starts receiving a load of the slide 2. At this time, a current value of the servomotor 16 is changed. When this current change is detected, the controller 100 obtains a cushion pressure based on the current value, and controls the servomotor 16 so that the obtained cushion pressure follows the pressure pattern of the preset cushion pressure. Then, the cushion pad 11 lowers while generating an upward energizing force, and reaches a bottom dead point.
An error of the control affects to throttling workability, and causes the work 4 to be broken or strained. Therefore, the controller 100 needs to control the operation of the cushion pad 11 so that the obtained cushion pressure follows to the set pressure pattern.
With respect to an accuracy of the operation of the cushion pad, the above-mentioned Japanese Patent Application Laid-Open No. 10-202327 has a problem. Generally, a feedback control must be a closed loop for measuring a physical amount in a control object and controlling the control object based on its measured value. If the feedback control of the cushion pressure of the cushion pad is performed, it is necessary to measure the load generated in the cushion pad.
However, in the above-mentioned Japanese Patent Application Laid-Open No. 10-202327, no physical amount is measured from the cushion pad side, and the current value of the servomotor for driving the cushion pad is merely measured. Though the load generated in the cushion pad and the current value of the servomotor have a certain relative relation, but it cannot be said that they always have a predetermined relationship. Therefore, it is severely said that the feedback control of the above-mentioned Japanese Patent Application Laid-Open No. 10-202327 does not become a closed loop. In the technology of the Japanese Patent Application Laid-Open No. 10-202327 from such a point, there is possibility of being not able to accurately control the operation of the cushion pad. In the worst case, the work generates a breakage or a strain.
The present invention is made in view of the above-mentioned circumstances, and aims to solve the problem by performing the feedback control of the cushion pressure in a closed loop and controlling a cushion pressure of a cushion pad in high accuracy.